Chelates constants

The Ee " chelating properties of exochelin MN (17) were investigated in detail (pK values, chelation constants, redox equilibria, etc.) (87). In one publication (128) siderophores from Mycobacterium tuberculosis otherwise referred to as carboxymycobactins (see below Sect. 2.8) were also named exochelins. 

A comparative study of the chelation of 2-deoxy-D-ribose with boric, germanic, telluric, and arsenious acids using potentiometry has shown that only 1 1 complexes are formed. Values of the chelation constants were compared with those of the pentoses. A similar study on mannitol and glucose with boric acid demonstrated that the former was about 740 times more effective a complexant than glucose, but that the conclusions were complicated by the degree of association of boric acid in solution. Reactions at the anomeric centre of mannofuranose have been carried out by means of the 2,3 5,6-ethylboronate (1) formed by treatment of D-mannose with limited triethyl boroxine. By the same means D-lyxose gave the 2,3-blocked lyxofuranose (2) also useful for... 

Calcium gluconate acts like a fluoride ion chelator. But its chelating constant is weaker than that of the HEXAFLUORINE solution. 

The stability constants of the species formed, which may be chelation constants, ion-exchange constants, or, in the case of precipitation, solubility products. 

The first identified complexes of unsubstituted thiazole were described by Erlenmeyer and Schmid (461) they were obtained by dissolution in absolute alcohol of both thiazole and an anhydrous cobalt(II) salt (Table 1-62). Heating the a-CoCri 2Th complex in chloroform gives the 0 isomer, which on standirtg at room temperature reverses back to the a form. According to Hant2sch (462), these isomers correspond to a cis-trans isomerism. Several complexes of 2,2 -(183) and 4,4 -dithiazolyl (184) were also prepared and found similar to pyridyl analogs (185) (Table 1-63). Zn(II), Fe(II), Co(II), Ni(II) and Cu(II) chelates of 2.4-/>is(2-pyridyl)thiazole (186) and (2-pyridylamino)-4-(2-pyridy])thiazole (187) have been investigated. The formation constants for species MLr, and ML -" (L = 186 or 187) have been calculated from data obtained by potentiometric, spectrophotometric, and partition techniques. 

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

In common with other hydroxy organic acids, tartaric acid complexes many metal ions. Formation constants for tartaric acid chelates with various metal ions are as follows Ca, 2.9 Cu, 3.2 Mg, 1.4 and Zn, 2.7 (68). In aqueous solution, tartaric acid can be mildly corrosive toward carbon steels, but under normal conditions it is noncorrosive to stainless steels (Table 9) (27). 

For thermodynamic (stabiUty constants) and kinetic data involving crown-type inclusion complexes see References r38 and r39 stmctural results in References r40—r42 (see also Chelating agents). 

Chelation is an equilibrium system involving the chelant, the metal, and the chelate. Equilibrium constants of chelation are usually orders of magnitude greater than are those involving the complexation of metal atoms by molecules having only one donor atom. 

Experimentally deterrnined equiUbrium constants are usually calculated from concentrations rather than from the activities of the species involved. Thermodynamic constants, based on ion activities, require activity coefficients. Because of the inadequacy of present theory for either calculating or determining activity coefficients for the compHcated ionic stmctures involved, the relatively few known thermodynamic constants have usually been obtained by extrapolation of results to infinite dilution. The constants based on concentration have usually been deterrnined in dilute solution in the presence of excess inert ions to maintain constant ionic strength. Thus concentration constants are accurate only under conditions reasonably close to those used for their deterrnination. Beyond these conditions, concentration constants may be useful in estimating probable effects and relative behaviors, and chelation process designers need to make allowances for these differences in conditions. 

Table 2. Concentration Formation Constants of Metal Chelates...

If the metals or ligands involved in a displacement reaction form chelates where type formulas are different, the exchange equiUbrium constant is the simple ratio of the formation constants of the chelates. Rather, for the reaction... 

In both cases n is the number of hydrogen ions displaced in the formation of the complex. In solutions where the ratio of free chelating agent to complex, ], is held constant, the slopes of the curves pM vs pH are equal. o n m. the region where H A is the principal form of the chelating... 

The more stable the chelate, the higher the pM that it can maintain, and the higher the pH required to precipitate the metal hydroxide. From equation 22 it can be seen that the smaller the solubihty product ie, the more iusoluble the metal hydroxide, the higher the pM that a chelant must maintain to prevent precipitation. The stabiUty constant of the Fe(III)—EHPG complex (/2), is so large (10 ) that iron is not precipitated even ia strongly alkaline solutions. 

In the equation for pM, log K appears instead of piC because iCis a formation constant, the reciprocal of the chelate dissociation constant, which is analogous to the acid dissociation constant K. ... 

The product is equal to the equilibrium constant X for the reaction shown in equation 30. It is generally considered that a salt is soluble if > 1. Thus sequestration or solubilization of moderate amounts of metal ion usually becomes practical as X. approaches or exceeds one. For smaller values of X the cost of the requited amount of chelating agent may be prohibitive. However, the dilution effect may allow economical sequestration, or solubilization of small amounts of deposits, at X values considerably less than one. In practical appHcations, calculations based on concentration equihbrium constants can be used as a guide for experimental studies that are usually necessary to determine the actual behavior of particular systems. 

Chelation is a feature of much research on the development and mechanism of action of catalysts. For example, enzyme chemistry is aided by the study of reactions of simpler chelates that are models of enzyme reactions. Certain enzymes, coenzymes, and vitamins possess chelate stmctures that must be involved in the mechanism of their action. The activation of many enzymes by metal ions most likely involves chelation, probably bridging the enzyme and substrate through the metal atom. Enzyme inhibition may often result from the formation by the inhibitor of a chelate with a greater stabiUty constant than that of the substrate or the enzyme for a necessary metal ion. 

The log function of the ratio of chelated metal ions to free-metal ions is expressed as the stabiUty constant or formation constant as shown in Table 6. The higher the stabiUty constant the greater the percentage of metal ions that are chelated (11). 

Ethyl Acetate. The esterification of ethanol by acetic acid was studied in detail over a century ago (357), and considerable Hterature exists on deterrninations of the equiUbrium constant for the reaction. The usual catalyst for the production of ethyl acetate [141-78-6] is sulfuric acid, but other catalysts have been used, including cation-exchange resins (358), a- uoronitrites (359), titanium chelates (360), and quinones and their pardy reduced products. 

C and H chemical shifts and the corresponding coupling constants have been determined for the chelate (225 M = Zn(II)) (81M105). 

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